feat(std/node): Add support for process.on("exit") (#8940)

This commit adds support for process.on("exit") by appending a listener to
the unload event. Luckily, unload works pretty much the same as on("exit")
since it won't schedule any additional work in the even loop either.

This commit also solves an error in the Node implementation, since "process.argv"
didn't contained the main module route as it was supposed to.

std/node/global.ts

@@ -1,6 +1,6 @@
 // Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
 /// <reference path="./global.d.ts" />
-import { process as processModule } from "./process.ts";
+import processModule from "./process.ts";
 import { Buffer as bufferModule } from "./buffer.ts";
 import timers from "./timers.ts";